The present invention relates to a hydraulic machine having a Francis type runner such as pump and pump turbine and, more particularly, to a system for operating such a type of hydraulic machine.
The hydraulic machine of the kind stated above has a runner carried by a shaft which in turn is directly connected to a motor or a motor generator. The runner is accommodated by a casing into which the water to be pumped is introduced through guide vanes and stay vanes.
If the hydraulic machine is the pump turbine, it operates also as a water wheel. In such a case, the runner is driven by water introduced from the casing through the guide vanes and stay vanes, to drive the motor generator which in this case produces the electric power. Thus, in the Francis type runner streams of water are formed in one and the other flowing directions depending on whether the hydraulic machine is used as a pump or a turbine.
As well known to those skilled in the art, the Francis type runner has a crown ring constituting the upper part of the runner, a shroud ring constituting the lower part of the runner and having a central bore presenting the passage for the water, and a plurality of blades disposed between these rings. As the water head becomes greater, the ratio of the radius of the runner to the height of the runner is increased so that the runner has a generally flattened form. When the pump turbine is used as a pump, water is sucked into the runner from a draft tube which openes to the lower side of the runner, through the opening formed in the shroud ring, and is then discharged in the horizontal direction from the periphery of the runner. To the contrary, when the pump turbine is used as a turbine, water flows in the horizontal direction into the runner through the periphery of the latter, and is discharged into the draft tube from the center of the runner.
The periphery of the runner is opposed by a stationary member. Also, an upper cover is disposed above the crown ring, while a lower cover is disposed below the shroud ring. The lower cover has portions opposing to the periphery of the runner (the outer peripheral surface of the shroud ring) and to the inner peripheral surface of the shroud ring. A lateral pressure chamber is defined between the lower cover and the shroud ring. In the opening of the lateral pressure chamber, i.e. in the gaps between the shroud ring and the outer and inner peripheral surfaces of the lower cover, provided are seals constituted by sealing members opposing to each other with small clearance left therebetween. These seals are effective in diminishing the rate of the water leakage from the periphery of the runner to the draft tube through the lateral pressure chamber.
However, as the water head becomes large, the rate of leak of the water into the lateral pressure chamber becomes also large to the increase of the rate of the water leakage decreases the effective work of the pump turbine.
To overcome this, it is a current measure to constitute the inner seal by a plurality of stages. The reason why the inner seal rather than the outer seal is formed in multiplicity of stages is as follows. If the outer seal is formed in a plurality of stages, the increased thickness of runner for providing the multiplicity of stages will inconveniently increase the centrifugal force to require a greater mechanical strength of the runner. In both cases of the pump and turbine operations, a higher pressure is established at the outer peripheral portion than at the inner peripheral portion of the runner. Therefore, the water comes into the lateral pressure chamber mainly through the outer seal. The pressurized water flows also into a back pressure chamber defined between the crown ring and the upper cover, so that it becomes necessary to adjust the vertical thrust acting on the runner. To this end, it is necessary to provide a multi-stage seal also between the crown ring and the upper cover, which in turn requires a further increased strength of the runner.
Due to, for example, these reasons, the inner seal is constructed in a multiplicity of stages and the adjustment of thrust is achieved by leading the pressurized water from the back pressure chamber to the lateral pressure chamber. However, high pressure water in the lateral pressure chamber discharge to the lower side of the runner through the multi-stage seal and adversely affect the performance of the pump turbine particularly when the latter is operating as a pump.
More specifically the flow of water discharged into the draft tube through the multi-stage inner seal has a velocity component in the direction of rotation of the runner. As a result, the swirl in the same direction as the runner is applied to the pumped water, and the pumping performance deteriorates. Also, the vigorous eddy currents are formed in the area to which the pressurized water is discharged and the runner is undesirably eroded by the cavitation generated in the eddy currents. These drawbacks are all attributable to the discharging of the pressurized water through the multi-stage inner seal to the suction side of the runner during the pumping operation. Thus, these drawbacks will be eliminated by taking a suitable measure for preventing the pressurized water from being discharged through the inner seal, and erosion of the runner will be eliminated, too.
From this point of view, Japanese Patent Laid-open No. 12244/73 proposes a pump turbine in which the pressurized water is discharged from the lateral pressure chamber or from a chamber defined beneath the inner seal. However, when the pressurized water is discharged from the side pressure chamber, the pressure in the latter is inconveniently lowered to incur an increase of the downward thrust to cause an overload of the bearing supporting the shaft. Also, when the water is discharged from the chamber defined beneath the inner seal, it is necessary to discharge the pressurized water at a large rate so that the water discharged through the inner seal may not reach the inside of the draft tube. In order to discharge the water at a large rate, also the water in the draft tube is sucked and then discharged. Even if the water coming through the seal is solely discharged, the seal in this prior art is a single stage seal to permit the discharge of a large amount of water from the lateral pressure chamber. Thus, this measure is materially equivalent to the direct discharge from the lateral pressure chamber.
Meanwhile, the specification of U.S. Pat. No. 3,253,816 discloses a technic in which the mixture of water and air are discharged through the inner seal, during the pumping operation of the pump turbine. More specifically, in this prior art, the space around the rotary body (runner) is filled with air to reduce the friction of the fluid during the rotation of the runner. Thus, during the pumping operation, the lateral pressure chamber contains almost no water, and the water discharged through the seal is a part of the water which has leaked through the outer peripheral seal of the runner and a part of the water which has been introduced through the inner seal from the draft tube due to the centrifugal force. Therefore, this prior art has nothing to do with the problem of bad influence of the water discharged through the inner seal. In addition, the hydraulic machine to which this prior art is applied has no multi-stage seal.